Continuous metal fiber brushes

ABSTRACT

A conductive fiber brush including a brush stock composed of plural conductive fibers or strands of fibers at least some of which may have plural bends along the leg of the fibers or strands. The fibers may have a diameter less than 0.2 mm and are arranged in contacting engagement with each other with the touching points among the fibers or strands maintaining elastic tension between the fibers or strands and thereby maintaining voids between the fibers or strands to produce a packing fraction between 1 and 50% and in extreme cases up to 70% but generally between 10-20% depending on the various factors, including the materials used, the current densities to be conducted, and the sliding speeds under operation. The plural bends are implemented by producing fibers or strands having a regular or irregular spiral, wavy, saw-tooth, triangular, and/or rectangular pattern, or other undulating pattern. Optionally, the voids in brush stock may be partially filled with a strengthening, lubricating, abrasive, and/or polishing material, and may be wrapped in an outer sheath, slid into a casing, or provided with an other covering of all or part of the area of the brush stock, be infiltrated or sprayed at the surface with some material, have an increased packing fraction at the surface and/or have some or all of the touching points between the fibers or strands soldered, welded or otherwise thermally joined. Optionally also, the friction among the fibers may be reduced through light lubrication applied by rinsing the brush or brush stock in a lubricant. In one embodiment, the fiber brush is employed in a brush loading device having a hydrostatically controlled brush holder wherein the force exerted on the brush is controlled by a metallic or other conductive hydrostatic fluid which at the same time conducts the current to the brush.

This invention was made in part by funds provided by the U.S. Departmentof the Navy. The U.S. Government may therefore have certain rights inthe invention.

DESCRIPTION

1. Technical Field

This invention relates to fiber brushes, and in particular, theimprovements in the design and manufacture of fiber brushes of the typedisclosed in commonly owned U.S. Pat. Nos. 4,358,699 and 4,415,635, thedisclosures of which are incorporated by reference herein.

2. Background Art

Although graphite and metal-graphite brushes have for nearly 100 yearsdominated the field of electrical brushes, for many applications therenow exists a superior form of sliding electrical conduction; highperformance fiber brushes wherein typically the fibers are made of metalfor which reason they are called metal fiber brushes. Prime candidatesfor this new technology include sliding electrical systems which requirehigh current densities, high sliding speeds, low electrical noise, highefficiency (low brush losses), compact size, or long brush lifetimes.

In particular, low voltage electric motors and generators can be madesmaller, more powerful and longer lasting owing to the increased currentcapacity, higher efficiency and longer wear life. This has a directbearing on electric vehicular and ship drive systems as well as lowvoltage electrical power generators. Other applications which requirehigh currents, such as high-force linear actuators, electromagneticbrakes, and armatures, are similarly well suited.

Many signal-critical electronic devices such as rotating antennae, sliprings and shaft pickups for electronic sensors and other transducerscould greatly benefit from the low noise and low voltage dropcharacteristics of metal fiber brushes. In addition, the new generationmetal fiber brushes can be manufactured with dimensions as small asfractions of a millimeter with user-selected stiffness (as measured inapplied brush force in Newtons per millimeter of resulting brushcompression, for example), making them usable as closeproximity,multiple-pole sliding pickups. They are also superior for delicaterotating instruments, since the required brush forces are much lowerthan for typical graphite based brushes. The broad-band electrical“noise” emission spectra of electrical equipment such as drills, sawsand other power tools can be greatly reduced by the use of metal fiberbrushes, thereby reducing or eliminating the electrical interferencethrough these brushes in use near sensitive electronic equipment.

As an interface, metal fiber structures and material can provide a lowloss connection at greatly reduced forces, thereby providinghigh-efficiency, low force electrical contact. This is particularlyimportant for high-current, low voltage switching, such as encounteredin variable voltage battery storage systems which are charged at highvoltages. Based on simple laws of physics, the capability of fiberbrushes to efficiently transfer electrical current across interfaceswhich are in relative motion or at rest, is paralleled by theircapability to similarly transfer heat. Therefore the brushes can also beused as heat transducers for cooling or heating purposes. Theoutstanding features of metal fiber brushes and some suggestedapplications are listed as follows.

High Current Capacity

Because metal fiber brushes can operate at very low losses, andconsequently at low heat evolution rates, they can conduct highercurrent with lower losses than graphite based brushes. Continuouscurrent densities of over 310 A/cm² (2000 A/in²) have been demonstratedand this does not by any means represent an upper limit. Accordingly,equipment which operates at high currents and low voltages can be mademore efficient and in many cases can run at higher power levels.Examples of this type of equipment include homopolar motors andgenerators, which have applications in electric automotive, rail andship drives, low voltage generators, such as those used with fuel cellsand with such applications as the hydrolyzation of water for combustiblefuel production. Similarly, linear high current devices, such as linearactuators, and linear pulse generators.

Low Electrical Noise

As already mentioned above, metal fiber brushes can operate at muchlower electrical noise levels than traditional graphite-based brushes.This can have dramatic benefits for signal-critical equipment on twofronts. First, instrumentation which requires rotating or linear slidingcontacts, such as rotating antennae, can achieve much higher signalresolution than with graphite-based brushes. Second, machinery will giveoff much less electrical noise and therefore cause much less inducedinterference when located in close proximity to sensitive transducers,detectors, and other electronic equipment if metal fiber brushes areused.

Long Wear Life

Metal fiber brushes can achieve not only low dimensionless wear rates,measured in wear length of brush shortening per length of sliding path,but they can also be constructed with very long, and in some casesnearly unlimited, permissible wear lengths. This translates to extremelylong brush life and greatly lengthened service intervals. For example,metal fiber brushes have demonstrated a dimensionless wear rate of2×10⁻¹¹, and at this rate a brush will wear by 5 cm of wear length over2.5×10⁹ meters of sliding path, or over 1.5 million miles. Obviously,continuously operated equipment would greatly benefit from this featureof metal fiber brushes.

High Sliding Speeds

Many applications such as high speed motors and generators requireelectrical brushes which can operate at high sliding speeds. Metal fiberbrushes have been successfully operated at speeds in excess of 70 m/sand their theoretical limit certainly lies considerably higher thanthat.

Compact Size

Electronic systems which need close proximity to a moving power orsignal coupling, or spacecritical sliding contacts could be furtherminiaturized by the use of this new generation of metal fiber brushesbecause these brushes can be made in sizes down to fractions ofmillimeters in thickness or diameter. This has a particular applicationrelating to signal power, and control-line pickups from rotating shaftssuch as are found in satellites, aircraft, periscopes, or many kinds ofrotor testing systems.

Low Heat Dissipation

Because they operate at low loads and have very low resistance, metalfiber brushes dissipate much less heat than typical brushes inhigh-current or high-sliding-speed applications. This could be of greatbenefit in insulated or temperature sensitive equipment such asrefrigeration systems or devices that incorporate compact rotatingelectronics.

Clean Operating

Unlike graphite-based brushes, metal fiber brushes do not generate finecarbon dust, which can cause problems not only with appearance andclean-up but also with long-term fouling and shorting. Metal fiber brushwear debris is heavy enough to be easily trapped or filtered making ittherefore much easier to keep the system clean.

In addition, an advantage of metal fiber brushes is the smallerproduction of presumably more benign wear debris as compared to that ofgraphite-based brushes. At anticipated similar dimensionless wear ratesof conventional and metal fiber brushes, reduction of wear debris volumefrom the latter is due to smaller running areas on account of increasedcurrent densities in combination with the fact that typically 80% to 90%of the brush is voidage, (1−f) with f the “packing fraction” of thevolume occupied by fibers, which does not produce wear debris. Theextreme limits of packing fraction range between 1% and 90%.

DESCRIPTION OF THE INVENTION

a. General Considerations

The previous metal fiber brushes suffered from the following problems;

difficulty of manufacture

limitations on the achievable relationship between macroscopic brushstiffness and microscopic fiber compliance

problems associated with the necessity of using a removable constituentduring manufacturing

limitations on the types of metals usable as conductors in the brusheson account of the need for differential etchability or dissolution ofthe matrix material.

The ideal, therefore, are fibers assembled into the form of rods(brush-stock), typically but not necessarily straight and of constantcross section, which locally leave the fibers within them individuallyflexible such that the properties at the interface to the conductingsurface do not change if run end-on even for long periods of time so asto cause considerable wear.

b. General Characteristics of Brush Stock

The most important feature of fiber brushes is that at any one moment alarge number of fibers, electrically connected to a current supply orsink, touch the interface (the rotor or substrate) which is electricallyconnected to the opposite pole. This requires that the fiber ends are atleast somewhat independently mobile so as to be free to “track”thesubstrate contours. The efficient production of fiber brushes istherefore possible through the construction of “brush-stock”incorporating a multitude of electrically conducting fibers (preferablyof 0.2 mm diameter or less) in a mechanically stable arrangement, whichfibers extend along the brush stock for individual lengths not shorterthan the brushes to be cut from the brush stock, and are substantiallyevenly spaced with a packing fraction f ranging as high as 70% or as lowas 2% for special applications, but more typically varying between 10%and 20%. In the previous U.S. Pat. Nos. 4,358,699 and 4,415,635,otherwise comparable brush stock included a matrix material in which thefibers were embedded and which had to be etched away or dissolved inorder to expose the fibers. The present invention substitutes emptyspace, i.e. “voidage”, for such matrix material and the improvementswhich are necessary in order to accomplish this.

In principle, making such brush stock including voidage instead of amatrix material, requires the production of tows, felts, weavings,ropes, spooled layers or braids of fibers, in any combination, and toshape these into brush stock of a predetermined shape which withoutimposed forces includes a predetermined voidage and is mechanicallystrong enough to withstand the lengthwise brush pressures (typically upto a few newtons per square centimeter) without being crushed, and thebending forces on the brushes made from the brush stock which resultfrom the friction between brush and rotor or other substrate. It alsorequires means by which to cut the brushes from the brush stock andproducing working surfaces at which the fiber ends are individuallyflexible. Note, however, that high flexibility in regard to bending canbe an advantage in case long pieces of brush stock are guided throughsuitable “guides” or apertures, if desired arranged so as to be pushedforward against the contacting surface through their own internalstress, much like a constant-force spring.

Such brush stock is characterized by the common feature that its crosssection, or the cross section of its outer shell, is shaped to suit theintended application conditions of the brushes cut from it.

c. Fiber Materials

The basic requirement for the fibers is that they be electricallyconductive. This means that they also are good heat conductors and thatthe brushes may be used for heat transfer across interfaces in the samemanner as for current conduction. However, not all fibers within a givenbrush stock have to conduct current but some may have the purpose ofincreasing the mechanical stability of the brush (“support fibers”), andalso for various other reasons fibers of different materials, crosssectional shapes and diameters may be used in the same brush.

In applications involving high current densities, the fibers arepreferably made of the traditional metal conductors, specificallycopper, silver, gold and their various alloys including brasses, bronzesand monels as commonly used in technology. On account of low cost andlow intrinsic electrical resistivity, aluminum could in principle beuseful, especially for physically large brushes, but it is prone to ahigh film resistivity and cannot be commercially obtained in fiberdiameters thin enough for most purposes.

Under demanding conditions when cost is of little concern, besides gold,a variety of noble metal and metal alloys comprising silver, gold,rhodium, palladium and/or platinum in various proportions, a number ofthese which are available commercially, will be very useful. Forprotection from oxidation and corrosion of the base metals, platings ofthese noble metals are valuable. For use in conjunction with liquidmetals, especially the sodium-potassium eutectic which is fluid at roomtemperature, niobium fibers are superior and would be difficult toreplace. For commutating applications, prospects are good for cadmium orcadmium alloy fibers, and for use in rail transportation iron and itsalloys, i.e. steels, importantly among them stainless steels are useful.Further, for some purposes, e.g. tarnish resistance, reduction offriction, provision of a protective layer for the substrate or rotorsurface, wear rate reduction or facilitation of alloy shape fixing oreutectic bonding (see below) fibers are advantageously provided withsuitable platings, e.g. of copper, silver, nickel, gold or othersuitable metals or non-metals. Also, carbon/graphite may be used asfiber material and graphite or diamond plating can be invaluable forsome applications. Finally, especially at high temperaturessemiconductors could also be used, among them germanium and silicon.

d. Fiber Shapes, Internal Brush Friction

The cross sections of fibers will ordinarily be circular but they may bearbitrarily shaped, e.g. be elliptical, triangular, quadratic,polygonal, strip-like with or without curvature, and tube-like with oneor multiple bores and have arbitrary external cross sections, as may besuitable for different purposes. In particular, strip-like fibersoriented with their long axis parallel to the sliding direction mayfacilitate reversals of sliding direction during operation, and boresmay contain lubricants or be used for cooling purposes or delivery ofcover gas. Also required are means to establish and maintain a desiredfairly uniform distribution of the fibers at a predetermined packingfraction.

e. “Interior” Strengthening Through Eutectic Bonding and/or Alloy ShapeFixing

Often, especially at low packing fractions as may be desirable in orderto conserve costs in case of noble metal fibers, one may want to makethe brush stock stiff largely without regard for internal friction. Infact, the brush stock can be greatly strengthened by setting thetouching points, or joints, in place through local soldering or welding.According to the present invention this is accomplished particularlyeffectively through “eutectic bonding”. Stiffening of the brush stockwithout increasing internal friction is accomplished through “alloyshape fixing”, wherein the momentary shape of the fibers is set intoplace through annealing at or above the recrystallization temperature.

f. Surface Treatments

The inventors realized that a rod-like, tube-like or strip-like fiberassembly as discussed would perhaps not necessarily need, but wouldmostly benefit from, some “surface treatment” to counteract the tendencyfor unraveling of the fibers about the circumference and at the rotorsurface. “Surface treatments” include any and all treatments which willjoin the peripheral fibers more firmly together than interior fibers orto provide some kind of strengthening “skin”. The effect of such surfacetreatments is to protect the macroscopic brush shape against splayingapart under the applied lengthwise force, preventing fibers at thesurface to fluff out or unravel, and to increase the resistance of thebrush stock against imposed forces, e.g. bending on account of frictionagainst the tangentially moving rotor surface.

Surface treatments can take the form of an external casing of a materialor geometrical construction different from that of the rest of the brushstock, into which the fibers are inserted or which is formed about thefibers. A surface layer can be applied through some treatment of theoutermost layers of fibers, e.g. through spraying onto the brush stock amaterial which hardens. A sheath can be applied through wrapping thebrush stock with a suitable foil or with metal leaf, with or withoutsubsequent heat treatment to induce eutectic bonding and/or alloy shapefixing (see below) on the surface layers. Alternatively, surfacetreatments may be applied through rolling in a powder or slurry, throughdipping in a liquid, or through electro-deposition or electrolessdeposition. Specifically, eutectic bonding can be used for surfacestiffening via any application of Sn or In in conjunction with silver,copper, silver alloy and copper alloy fibers. It can be accomplished,for example, by wrapping the fiber bundles (in previous experiments ofCu or Ag or brass) with an outer sheath of copper or brass foil linedwith an Sn or In foil. The sheath is then essentially soldered to thefibers on heating to the melting temperature of the Sn or In.

g. Partial or Complete Filling of Voidage

For the further improvement of fiber brushes the inventors had envisagedto mix graphite with the fibers to provide a lubricating and protectivefilm for use in the open atmosphere. However, problems have beenencountered with the intended admixture of graphite powder in theprocess of brush stock manufacture since it interferes with the eutecticbonding of silver and copper. However, graphite can be injected into thebrushes as a slurry after completion.

h. Brush Loading

A further consideration in the use and operation of metal fiber brushesis the mechanical loading applied to the brushes during use. Metal fiberbrushes can conduct very high current densities but require much lightermechanical loading than conventional, “monolithic” brushes. Moreover,the brush force has to remain constant within reasonably close,predetermined limits, independent of the length of brush wear. Thiscauses a problem because 1), the constant-force springs widely used forconventional brushes have a much too high electrical resistance for thepurpose, especially if they are designed for low loads, and 2),conventional current leads capable of conducting the required highcurrents to and from the brushes, are stiff and interfere with theintended light mechanical loading. Furthermore, for practical massapplications, fiber brushes will eventually have to be sold/distributedin a packaged form which protects them from damage during storage,shipment and handling, and which is designed for fool-proof installationby private persons or unskilled workers, much like light bulbs orprinter cartridges.

U.S. Pat. No. 4,415,635 envisaged metal fiber brushes composed ofhair-like metal fibers protruding from a matrix material and conductingcurrent to an electrically conducting surface (typically in relativemotion to the brushes) against which the fiber ends were lightly,mechanically pressed. U.S. Pat. No. 4,358,699, greatly elaborated ondifferent possible configurations of the concept of using hair-finewires in electrical brushes, including the fibers contacting theconductor along their long surfaces, being felted or woven together, andstrengthened in various manners, including by the incorporation of“support fibers”, being fibers which are substantially more rigid and ofa length a little shorter than the average fibers so as to protect thesefrom accidental damage. The drawback of other than end-on contactbetween fibers and opposing conducting surface is too short a wear-life.Namely, wear by one fiber diameter shortens a fiber little if it occursend-on but cuts off a whole length of fiber if it occurs on a lengthwisesurface.

Disclosure of the Invention

Accordingly, one object of this invention is to solve the problemsassociated with the prior art metal fiber brushes.

A further object of this invention is to provide a new and improvedelectrical fiber brush stock from which electrical brushes can be cuthaving low electrical contact resistance, and associated therewith lowinterfacial heat generation and a low sliding wear rate.

A further object of this invention is to provide novel fiber brushes inwhich, at the interface to the conducting surface, the fibers areindividually flexible.

Yet another object of this invention is to provide a new and improvedmethod of manufacturing metal fiber brushes.

Yet another object of this invention is to provide a fiber brush thathas a long wear life and does not change its characteristics throughwear.

Another object of this invention is to provide a fiber brush which iscompact in size.

Yet another object of the invention is to provide an electrical brushwhich emits little electrical noise.

Yet another object of the invention is to provide an electrical metalfiber brush which can be used with high current densities.

Still a further object of this invention is to provide a new andimproved brush holder and loading device which maintains constant brushforce while the brush wears.

These and other objects are achieved according to the present inventionby providing a new and improved metal fiber brush including a brushstock having plural conductive elements and a cross section shaped inaccordance with the intended use of the fiber brush. Some of the fibersmay have plural bends along the length thereof. In addition, there isprovided a new and improved method of making a conductive fiber brushincluding providing fibers, and bundling the fibers into a brush stockin which the fibers are in contacting engagement with each othermaintaining voids between the fibers. This can be accomplished by meansof a suitable die or form, within which the fiber arrangement concernedis constrained, or compressed, or into which it is permitted to expand,so as produce the desired cross-sectional form of the brush stock. Thebrush stock shaping may in commercial production be replaced orcomplemented by extrusion, continuous rolling or other reshapingmethods, all while producing the final desired voidage.

According to yet another aspect of the present invention, there isprovided a hydrostatically controlled brush holder mounting a conductivebrush, and a conductive hydrostatic fluid coupled under pressure to thebrush holder to control the force application to the brush as well aslead the current to it.

Still another aspect of the present invention, there is provided a brushholder which uses the elasticity of the brush stock to guide the brushstock forward against the contacting surface.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1a is a schematic side view illustrating a use of the fiber brushaccording to the present invention. FIG. 1b illustrates a strip-likefiber inclined to the substrate surface in the plane normal to thesliding direction;

FIG. 2 is a schematic illustration of a kinked fiber mass of a brush ofthe present invention showing the multiple touching points caused by thekinking, waving, spiraling, etc. These touching points cause elasticstresses which tend to keep the fibers from bunching together duringsliding, and also serve as possible bonding sites;

FIGS. 3a and 3 b are side and end views, respectively, of one possibleembodiment of an electrical fiber brush made using kinked fibers. FIG.3c is a perspective view of the casing surrounding the fibers in FIG.3b, and FIG. 3d is a perspective view of triangular casing. Typically,but not necessarily, a casing consists of a bonded kinked metal fibers.FIG. 3e shows a sheath in the process of being applied through wrappinga foil strip of Width Ds about the cylindrical brush stock at aninclination of angle y against the brush stock axis. Instead of a foil,the sheath can consist of a wrapping of fibers or, conversely, a widefoil, or any combination of these. FIG. 3f shows the cross section of arectangular brush stock including a surface layer which might have beenmade through dipping the brush stock into a suitable medium or sprayingit. Alternatively, the surface layer could have been formed by arrangingthe fibers near the brush stock surface to be more densely spaced thanfor the average of the brush stock or to be more strongly kinked, or thejoints to be bonded by any means including irradiation, electrophoresis,or electroplating. The non-sliding end of the brush can be soldered to amounting plate or stub, plated solid, or crimped to create the finishedassembly.

FIGS. 4a-4 k show examples of the different types of fiber bending asregular spiraling, irregular spiraling, regular waving, irregularwaving, curling, regular saw-tooth, irregular saw-tooth, rectangularbending, regular V-crimping, irregular V-crimping, and waving withintervals, respectively. FIGS. 4l and 4 m are illustrations of wavedfiber strands, one containing three, the other four fibers. FIG. 4nshows a twisted fiber strand which may be composed of two or moredifferent metals each of which the brush stock may be composed partly orwholly. FIG. 4o shows two different twisted strands which are twistedtogether;

FIG. 5a is a three-dimensional view of a piece of brush stock whosecross sectional shape is of a truncated triangle. FIG. 5b is asemi-schematic cross sectional view of the possible arrangement ofparallel spiral-shaped fiber strands of which the brush body in FIG. 5acould be composed. FIG. 5c shows nested concentric spiraled fibers ofwhich the brush stock of FIG. 5a could be composed in the arrangement ofFIG. 5b. FIG. 5d shows a brush stock in the form of a wavy strip;

FIG. 6 illustrates “support fibers” as first introduced in U.S. Pat. No.4,358,699;

FIG. 7a is a schematic cross-sectional view illustrating a novelmechanical loading applied to the metal fiber brush of the presentinvention. FIG. 7b and FIG. 7c show two different embodiments of loadingdevices using flexible brush stock of the present invention. FIG. 7dshows a guide used to guide a free end of the brush stock in FIGS. 7band 7 c;

FIG. 8a illustrates accordion-pleated layer of fibers or fiber felt.FIG. 8b shows a possible casing or sheath or other surface layer for theaccordion-pleated brush stock in FIG. 8a as compacted into the form ofFIG. 8c. FIGS. 8d and 8 e show alternative arrangements of pleats;

FIG. 9 illustrates production of fiber or strand layers by winding, forfuture rolling up or pleating;

FIG. 10 illustrates a method of stitching used to stiffen the brushstock;

FIGS. 11a and 11 b illustrate other forms of making a brush stock of oneor more layers of fibers, strands, or felt; and

FIG. 12a illustrates production of fibers or strand layers by winding,similar to FIG. 9, for making nested concentric spiraled fibers. FIG.12b illustrates the direction of the fibers or strands in FIG. 12a. FIG.12c illustrates rolling up a layer of fibers or strands into acigarette-shaped brush stock to yield nested spirals all of the samehandedness, e.g., left-handed. FIG. 12e illustrates the same method asin FIG. 12c but illustrates using two layers of opposite inclination(with arbitrary inclination angles labeled α and β) so as to yieldnested concentric spirals of alternating handedness, and FIG. 12dillustrates an example of a cigarette-shaped brush stock resulting fromrolling up a layer of fibers or strands as in FIG. 12c.

BEST MODE FOR CARRYING OUT THE INVENTION

General

The present invention provides metal fiber brushes which at the slidinginterface operate in the same manner as previously patented metal fiberbrushes but which, unlike those, are not painter's style but are cutfrom indefinite lengths of “brush stock” in the shape of rods or stripsof arbitrary cross section, and which after shaping and/or surface orother treatment as described hereinafter have a running surface readyfor use (in contrast to the prior art which required matrix material tobe removed from among the fibers). The brush stock is composed ofsubstantially parallel fine metal fibers (of diameter ≦0.2 mm, mosttypically about 50 μm within a range of 25 μm to 100 μm) whose lengthsare at least several millimeters and more typically extend through asubstantial part of the brush stock if not its whole length. The fibersare constructed so as to preserve, through potentially unlimited wearlengths, the characteristic metal fiber brush running surface, beingcomposed of a multitude of individually flexible fiber ends. It is thisstructure of the running surface which, provided the film resistivity(i.e. the resistance of unit area of film, a critical quantity) is low,conveys the desirable metal fiber brush properties of (i) low electricalcontact resistance, (ii) low electrical noise, (iii) ability to run athigh speeds, (iv) ability to be used at high current densities, and (v)ability, indeed need, to run at light mechanical pressure and thus lowmechanical loss; in all of these respects greatly outperformingconventional graphite-based brushes. While in most cases the fibers willbe made of metal, in some cases they may be of carbon (graphite) or ofsemiconductors such as germanium and silicon, especially if operation athigh temperatures is desired. For example, FIG. 1 shows a schematic sideview of a brush (1) in a typical working mode. The brush (1) has anindefinite length, an interface at the rotor or other substrate (4), andan surface layer or casing (10).

At the sliding interface, during use, the brushes contact the side towhich electrical contact is being made (the “rotor” or “substrate”) viaa multiplicity of individually moveable fiber ends. Although FIG. 1aschematically shows the fiber brush with a normal orientation to therotor surface, typically a brush is oriented at an arbitrary angle tothe rotor surface (e.g., 15°-20° in trailing orientation and/or up to,say, 45° in the plane normal to the sliding direction) with the brushshaped to assure continuous contact with the rotor surface. As anotherexample, FIG. 1b shows a strip-like fiber brush (8) in a working mode,wherein it is inclined to the substrate surface (4) in the plane normalto the sliding direction.

The requisite low wear rate of the brushes depends on running them withelastic contact spots under access of moisture (as is normally presentin the free atmosphere and otherwise must be provided). If the simplefiber brush theory holds true, to this end the brush pressure must bebelow p_(trans)≈3×10⁻⁴ fH, where p_(trans) is the critical force at thetransition between elastic and plastic contact spots, f is the packingfraction, i.e., fraction of metal in the brush volume, and H is theMeyer hardness of the fiber material (see eq. 10b of “Electrical FiberBrushes—Theory and Observations”, D. Kuhlmann-Wilsdorf, ICEC-IEEE Holm95, 41st Holm Conference on Electrical Contacts, IEEE, Montreal, Canada,Oct. 2-4, 1995, pp. 295-314; reprinted as “Electrical FiberBrushes—Theory and Observations”, D. Kuhlman-Wilsdorf, IEEE Trans. CPMTPart A, 19 (1996) pp. 360-375, which is incorporated by referenceherein). Preferably the brush pressure is p=βp_(trans) with ¼<β<½ which,under otherwise proper running conditions, will lead to dimensionlesswear rates in the 10⁻¹¹ range (see FIG. 2 in the cited paper). The brushpressure is adjusted so that the typical contact spot(s) between anysingle fiber and the rotor is/are only elastically, but not plasticallydeformed. That condition of elastic contact spots depends on a low loadper individual fiber and is attained at β<1. Correspondingly, very finefibers are desirable, and as discussed above are typically less than 0.2mm thick. If the condition of elastic contact spots is met, both theelectrical contact resistance and the sliding wear rate are low, as isessential for superior electrical brushes. (The described nature of thesliding interface is the same as for the previously patented brushesexcept that the role of adsorbed moisture was not yet known). Inaddition, for high current densities and high sliding speeds the optimumpacking fraction range, at time of writing is between 12-15% for brushesmade in the laboratory but, in agreement with the appended paper, it isanticipated that it will be near 20% in commercial production.

Preferred Fiber Materials

All conductive materials which can be formed into fibers are potentialcandidate materials for fiber brushes. Preferred choices include thetraditional technological metal conductors, including copper, silver,gold and their alloys, including among the copper alloys, brasses,bronzes and monels, all of the named metal fiber choices with andwithout platings, among these in particular gold, silver and nickel.Also preferred materials are niobium, rhodium, platinum, and in generalnoble metal alloys such as are commercially available for operatingelectrical contacts in the open atmosphere, among them Paliney alloys.Further, carbon (graphite) and semiconductors including germanium andsilicon are preferred materials. The choice depends on purpose,serviceability and cost; e.g. gold, platinum and rhodium are excellentfiber materials for almost all purposes but are very expensive andrhodium (and the harder noble metal alloys) tend to cut the rotor orother substrate surface. Among the noble metals, palladium is apreferred replacement for gold because it is lighter and much lessexpensive per troy ounce, with the further advantage that it plates wellon other metals. As a major drawback, according to best previouslaboratory experience, palladium tends to catalyze the formation ofcontact polymers which, if present, raise the film resistivity to anunacceptably high level. Niobium is almost irreplaceable for use inconjunction with liquid NaK. Nickel and nickel alloys are very corrosionresistant and have excellent mechanical elasticity. Further, nickel asan under-plate serves to prevent the diffusion of thin gold platings, inparticular, but also a number of other platings, into the underlyingcopper. Semiconductors such as germanium and silicon are potentiallyvaluable at high temperatures (in that case probably for highcostapplications with hard rotor surfaces such as rhodium or platinum groupalloys) but no experience with these does as yet exist, albeit iridiumhas been successfully tried on a very small scale. In addition, researchis occurring on conductive plastic materials that may be used. The lowercost of plastic materials and their resistance against environmentalattack are expected to be major advantages of using conductive plasticmaterials in fiber brush stock.

Control of Brush Stock Strength Through Touching Points

As in the previous brushes, the individual movability of the fiber ends,on which the desirable action of the brushes depends, is achievedthrough the inclusion of “voidage” such that the fibers occupy only afraction (the “packing fraction”) of the macroscopic brush volume.Previously, this was attained through letting the fibers protrude from amatrix material, typically by a length which was on the order of 100times the fiber diameter. However, use of parallel fibers protrudingfrom a rigid matrix material a la a painter's brush has the disadvantagethat already relatively minor wear lengths (compared to the macroscopiclength of the brush) substantially change its running characteristicsand thereby cause relatively short brush life-times.

According to the present invention, empty space, i.e. “voidage”, issubstituted for matrix material and the proper packing fraction, “f”,may be controlled by providing bends in the individual fibers along thelength of the fibers, e.g., by crimping, kinking, waving, spiraling orcurling the fibers in a regular or irregular pattern, so as to impart“loft”. This facilitates the desired fairly uniform distribution of thefibers and the desired constant packing fractions which are maintainedin spite of compressive forces in use. The effect is due to theestablishment of touching points (or “joints”) as shown, for example, inFIG. 2 where fibers touch mechanically, e.g. neighboring substantiallyparallel fibers, or mutually inclined fibers at crossing points. Forotherwise same fiber morphology and arrangement, the average spacing ofthe touching points along each fiber is controlled by the manner ofdistorting the fibers; for example as is shown in FIGS. 4a-4 k, thefibers can be modified through bending, kinking, curling, spiraling,waving, etc., alone or in any combination, with the bending or kinkingimparting arbitrary shapes with arbitrary amplitude and wavelength.

The conductive elements have contacting engagements with each other atirregularly longitudinally spaced contact points with the contactingengagements maintaining elastic stresses between the conductive elementsand maintaining irregularly longitudinally extending voids between theconducting elements.

A further tool in the construction of brush stock is the use of fiberstrands in lieu of or in combination with individual fibers. Fiberstrands are any bundled or twisted groupings of two or more fibers whichare used together, e.g. taken off one spool. A major advantage of theuse of strands is the increased speed of brush stock construction,resulting in cost savings. Another advantage of strands is that they canbe employed as a further means to control the density and nature of thetouching points in the brush stock. The fibers in any one strand are notnecessarily all of the same size, morphology or material. FIG. 4l showsa bundled fiber strand composed of three individual similarly wavedfibers and FIG. 4m shows a strand containing four fibers. A fiber strandmade through twisting of either individual fibers or of fiber strands isshown in FIG. 4n.

The effect of deviations from linearity of the fibers is to impart“loft” in much the same way as is the case for hair or textile fibers.This is due to an increase of “touching points” or “joints” among thefibers. The number of touching spots increases with the number of bendsper unit length of fiber or strand as well as their amplitude, i.e. themagnitude of the deviations from linearity. The number of touchingpoints or joints decreases with the number of fibers per strand.Geometrically a predetermined distribution of fiber joints may beobtained through twisting of two or more fibers together into twistedstrands as is shown in FIG. 4o, which may be further processed likesingle fibers, e.g. be bundled, spooled, or layered, or if desired twoor more bundled or twisted strands may be twisted together once againand the process repeated at will to effect roping. In this way a furthercontrol of the density and distribution of touching points, e.g. amongfibers of different materials, diameters or shapes, is achieved. Or elsepredetermined touching spots can be achieved through bundling, orarranging into layers, fibers which have been curled, waved or kinked inany way.

If desired, a roughly uniform distribution of touching points isachieved through regular self-contained elastic stresses. One examplehere is weaving and braiding of straight fibers. The same effect with alower density of touching points is obtained in brush stock in the formof a set of nested, graded concentric spirals, for example as is shownin FIG. 5c, made of intrinsically straight fibers, with either the sameor alternating sense of rotation from the center outward, or anyarbitrary sequence of sense of rotation. Brush stock which is composedof spirals with only one sense of rotation will, on brush forceapplication, tend to twist about the lengthwise axis. This effect isavoided when employing alternating handedness of spiraling as achievedthrough the method of FIG. 12e. Similarly, brush stock may be composedof cells of single or nested spirals as is shown in FIG. 5b, or in arelated geometry the fibers may be loosely roped for obtaining a lowdensity of contact spots. Crimping, kinking, waving, etc. of the fibersin any of these geometries increases the density of touching pointscorrespondingly.

Control of Internal Brush Stock Friction

While the effect of the touching spots is to keep fibers apart throughnormal forces at them, thereby aiding in the even distribution of thefibers and mechanically stiffening the brush stock, at the same timethrough local friction the touching points impede lengthwise relativemotion between the fibers and thereby interfere with the desiredindividual fiber-end mobility needed for tracking the substrate contour.Those undesirable internal friction forces which interfere withfiber-end mobility rise with the number of touching spots as well as theaverage force with which the fibers are pressed together. Both of theserise with packing fraction. Therefore in practice the upper limit of fis controlled by the degree to which proper brush operation depends onindividual fiber end mobility, e.g. higher f's may be used at low speedsrather than at high speeds, for smooth rather than for rough substrates,for high brush pressures rather than for low brush pressures.

It may be noted that the advantage of any of the geometries involvingspiraled or roped. fibers introduced above is that they exhibit reducedinternal friction on account of relatively few touching points, incombination with high reversible compressibility in lengthwisedirection. The latter is advantageous because it facilitates “tracking”of the fiber ends on the substrate. The brush stock stiffness againstbending depends on specific construction and is evidently low for ropingand much higher for the spiral cell structure.

Lack of stiffness against bending is not necessarily a disadvantage butrequires that brushes be guided through apertures which fix theirposition relative to the contacting surface at a distance whichdecreases with increasing brush stock flexibility in bending.

Given a certain morphology of the fibers, e.g. kinked or waved in aparticular manner to impart “loft”, the packing fraction may still bevaried independently, and with increasing f as well as “loft”, themacroscopic stiffness of the brush increases. Simultaneously, theability of the average fiber tip to remain in contact with the rotorsurface diminishes on account of the increasing number of, andincreasing forces at, the three-dimensional connections among thefibers, i.e. the touching points, either through rigid or frictionalbonding, as “joints” which are distributed along the fibers so as toleave some average free fiber length between them which shrinks withincreasing packing fraction.

In line with these considerations, it is often useful to reduce thecoefficient of friction at the average touching points so as to reducethe friction among the fibers and thereby improve individual fiber endflexibility as well as the length-wise elastic compressibility of thebrush stock. This can be done through rinsing with a lubricant. Adiluted colloidal graphite solution has been found to be very suitablein this regard. Even minute amounts of such lubrication, amounting onaverage to small fractions of 1 μm layer thickness on the fibers, havebeen found to be very effective to reduce internal brush friction, andalso to be capable of reducing the friction between the brush andsubstrate.

Shaping Brush Stock and Hardening Effect of Partial Filling of Voidage

Brush stiffness is increased by filling the void space (“voidage”, i.e.,the fraction (1−f) of the brush volume not occupied by fiber material)between the fibers wholly or partially with a suitable filler material.While this increases internal friction and for this reason is mostlyundesirable, the filler material may be chosen to serve as a lubricant,abrasive, polishing agent or other surface conditioner of the rotorsurface, to be further discussed below.

In any case, unless roped or spiraled, the brush stock is ordinarilyshaped via some mold or die. As a result, brushes according to thisinvention can have all of the same desirable characteristics as theprevious brushes but can be worn to indefinite lengths without change ofproperties.

As already indicated, the mechanical firmness of frictional bondingincreases with packing fraction as well as with the degree ofcurling/kinking and is thus controllable; e.g., for high packingfractions of very thin fibers (for high-performance brushes with verylow contact resistance), less curling or kinking will be used than forlow packing fractions (e.g., as for general purpose, low-cost brushes).Examples of the different shapes of a brush stock are shown in FIGS. 5aand 5 d. FIG. 5a shows a brush stock with a triangular shape and FIG. 5dshows a brush stock in the form of a wavy strip.

Methods for Internal Strengthen of Brush Stock

a—Eutectic Bonding

Brushes according to the previous invention, made from brush stockcomprising fibers embedded in a matrix material, had the additionaldisadvantage that the fibers tended to splay apart, exactly as thebristles in a painter's brush, if pressed down too firmly. Similarly,when pressed against the rotor or other moving surface, also brushesobtained from continuous fiber brush stock will splay apart and inaddition tend to bend. In order to prevent excessive bending and/or inorder to contain the fibers at the interface more or less within themacroscopic geometrical brush stock profile, the brush stock istypically stiffened at least at its perimeter. In the present inventionmechanical strength, most importantly against lateral extension orsplaying of the brushes during installation or use, independent of orbeyond that which may be achieved through control of touching points onaccount of friction among the fibers where they touch, or be due to afiller material, can be increased either through “interior bonding” (or“interior stiffening”) or through “surface treatment”.

“Interior stiffening”, throughout the volume of the brush stockindependent of void filling, may be effected through bonding of varyingdegrees of firmness at the touching points, or joints. Entirely rigidbonding may be obtained through what amounts to soldering or welding atthe joints via “eutectic bonding”. In this method a eutectic comprisingthe fiber, plating and/or stiffening material is allowed to form atabout and above the melting temperature of the eutectic. If the molteneutectic wicks into re-entrant corners at fiber touching points, theyare effectively soldered when the eutectic solidifies on cooling. Thecopper-silver eutectic, melting at about 800° C., is particularlysuitable for this method. Eutectic bonding requires physical touchingamong the constituents of the eutectic, e.g. takes place amongsilver-plated copper fibers, among copper-plated silver fibers, or amongmixed silver and copper fibers, or mixed fibers of any suitable alloysof these metals. A disadvantage here is that on account of the highmelting temperature of the silver-copper eutectic, the requisite highannealing temperature tends to destroy the “spring” of the fibers whichis needed for the elastic bending of the fiber tips in tracking thesurface profile of the opposing surface. Albeit this may be counteractedby the simultaneous alloy formation which is the basis of alloy shapefixing, especially if the annealing is followed by a quench (see AlloyShape Fixing below).

The low-melting (about 200° C.) eutectics of copper with tin or indiumdo not suffer from this disadvantage. However, they must be induced inrelatively high concentrations locally, say through a tin or indium foilembedded between fibers. This is for the reason that low-meltingeutectics tend to have a low surface tension (since thermodyamically thesurface free energy is roughly proportional to the melting temperature).Therefore, if layered on the higher-melting copper or silver, indium andtin remain spread rather than wicking into re-entrant corners andthereby exposing the copper or silver surfaces of higher energy. As aresult low-melting eutectics tend to only set joints which are wetted inthe course of forming the eutectic, meaning when a significant excess ofmolten eutectic exists before cooling. Further, the experiments made bythe inventors so far suggest that both Sn and In can leave a damaging,relatively high-resistance deposit on the brush track. This in turntends to cause over-heating whereupon the Sn (or In) melts and fuses thefiber ends together so as to make the brush surface stiff and causebouncing, effectively destroying the brush. It therefore seems, but hasnot yet been fully explored, that there exists a limiting concentration,depending on use of the brushes, above which tin and indium eutecticsshould not be used.

By the use of twisted strands comprising different metals, e.g. silverand copper in various proportions alone or together with bundled strandsor single fibers of either or both of the pure metals, the distributionand concentration of rigid bonds can be controlled within the interiorof the brush stock.

Instead of directly bonding fibers, one may also use metal powder mixedwith the fibers, e.g., silver powder with copper fibers or vice versa,in which case the eutectic soldering takes place between the powderparticles (which typically will dissolve or, at a high enoughtemperature, will melt in the process) and fibers which they touch. Inlieu of powders one may similarly intersperse metal foil or metal leafwith the fibers. All of these methods may be used together in anycombination, if desired involving different metals for the platings,powders and foils.

b—Alloy Shape Fixing

In case of very small concentrations of one of the two components usedin the process which otherwise leads to eutectic bonding, e.g. silverleaf on copper fibers, the treatment causes the “setting” of the fibergeometry and an apparent stiffening of the fibers inspite of the highannealing temperature used, even though optical microscopic examinationreveals no wicking of eutectic into re-entrant corners and the jointsare in fact not bonded at all. The inventors have concluded that (1)this mechanical stiffening of the fibers and (2) setting them into placeis due to two distinct effects which happen to occur simultaneously butcan in principle be used independently. Firstly, the mechanicalstiffening occurs through the diffusion of the low-concentrationconstituent (in this case the silver) into the fibers (in this case thecopper fibers), thereby forming the corresponding harder alloy.Meanwhile, simultaneously recrystallization took place to set the nowmuch stiffer alloyed fibers into the imposed “brush stock”configuration. Simple arithmetic suggests that in the present exampleonly the first, say, n<5 layers of fibers could have been so alloyed,which at, say, f=0.2 packing fraction, and d=50 mm fiber diameters witha net film thickness of t=2 mm could have given rise to a silverconcentration in the copper of c_(CU)=t/(fnd)≈4 vol %, i.e. enoughalloying to confer considerably increased strength to the fibers.Actually, it is questionable whether the alloying was uniformly spreadthrough the fibers, although with the speeding up of diffusion viaconcurrent recrystallization this could have been so.

The above leads to an improved method of forming fiber brush stock, viaannealing plated fibers or fibers mixed with metal leaf or metal powdersat their recrystallization or alloying temperature, whichever is higher,long enough to let some or all of the plating leaf or powder dissolve inthe fibers. This simultaneous alloying and recrystallization is expectedto increase the fiber strengthlelasticity while it sets into permanentplace the shape that is concurrently imposed on the fibers viacompressing in the brush stock form, or as rolled or twisted e.g. as inFIGS. 12d and e. Beyond this, the invention includes the possibility ofsimultaneously or subsequently using other metallurgical techniques,e.g. of establishing concentration gradients in the fibers, or quenchingand age-hardening, to improve the mechanical or other properties of thefibers. Also, setting into place may be done through heating to therecrystallization temperature independent of any diffusion treatmentand, conversely, diffusion treatments are possible below therecrystallization temperature and therefore without setting themomentary shape into place. It is conjectured that ordinary eutecticbonding, e.g. with copper fibers plated with a normal thickness ofsilver, did not lead to observed alloy strengthening because the liquideutectic layer was so thick that it quickly contracted into re-entrantcorners before significant diffusion of the lowconcentration constituent(e.g. silver) into the rest of the fibers could take place.Correspondingly, the optimal conditions for alloy shape fixing stillrequire exploration.

Suitable plated wires for alloy shape stiffening are expected toinclude: (i) copper-plated silver, (ii) silver-plated copper, (iii)nickel-plated copper, (iv) gold-plated copper with an under-plate ofnickel, to name those which are commercially available (i.e. (ii)) orcan be readily made even in our own laboratory. For maximum hardening atminimum loss of electrical conductivity, a zirconium plate on copper ora chromium plate on copper would be desirable. As implied by thepreceding explanations, it is anticipated that the plating thickness andannealing times can be adjusted to either yield an optimal alloy at fulldissolution of the plating material in the fiber (e.g. for (i) copperinto the silver so as to reduce oxide formation) or to leave a remnantplating as probably advantageous in the other three cases. Theparticular advantage of (iv) gold-plated copper with an under-plate ofnickel, is to harden the copper by means of the nickel, and retain agold-plate to lay down on the wear track a thin protective gold layer.Many other combinations are doubtlessly possible. Nor is the methodrestricted to two components, but three or more may be utilized, e.g.copper and silver may be diffused into gold alloy fibers, simultaneouslyor consecutively. Also non-metals can be employed, e.g. carbon can bediffused into iron or steel fibers.

c—Layering. Rolling-up or Pleating Fiber Layers or Fiber Felt

A disadvantage of interior eutectic bonding is that it raises interiorfriction. Other methods in lieu of or in addition to alloying throughdiffusion described in the preceding section may therefore be used tomechanically strengthen the bulk of the brush stock with lesser impacton internal friction. One method consists of placing a layer of fibersor strands, not necessarily all parallel, on a flat surface and rollingit up, as is shown in FIG. 11a, or folding or pleating it to the desiredshape of the brush stock. A fiber felt, consisting of a thin layer ofmutually misoriented fibers bonded at a suitable concentration oftouching points, can take the place of the layer of fibers. Similarly,one may layer fibers, felts and/or foils on top of each other and rollthem up. Likewise, as is shown in FIG. 8a, one may pleat the fibers,felts, and/or foils (13) into desired morphologies, e.g. by accordionpleating (14) parallel to the long axis of the brush stock, wherein theindividual fibers may be inclined at moderate predetermined angles, e.g.±30° to that axis. FIGS. 8c, 8 d and 8 e show alternative arrangementsof pleats to achieve different brush stock shapes.

Any of these methods strengthen the brush against bending even whileinternal friction may be kept low, depending on construction. Forexample, in lieu of or in addition to, internal eutectic bonding oralloy shape fixing, one may spread straight or kinked, waved, etc.,fibers and/or fiber strands out over a thin eutectically bonded skin, orover any suitable foil of, say, 0.1 mm thickness, and roll up theassembly (FIG. 11a) or fold it (FIG. 11b) appropriately into the desiredbrush stock shape. One may then either rely on the extra strengtheningeffect through the skin or foil, or one may with appropriate choice offibers continue with a eutectic bonding or alloy shape fixing heattreatment. In addition, in FIG. 8b, a possible casing (15) or othersurface treatment for accordion-pleated brush stock (1) with accordingpleats (14) may be made of foil or a layer of bias-oriented fibers orstrands, perhaps eutectically bonded as with any combination of Ag, Cu,Cu-plated, an Ag-plated fibers. Alternatively, one may interleave forexample copper fibers destined for the brush stock interior with silverleaf of only 1 μm thickness or less and use the alloy shape fixingtreatment. The requisite heating is such that the soldering and weldingmight be performed by rf induction heating, furnace heating or any othersuitable means.

Winding fibers or strands into layers for future rolling up or pleatingis illustrated in FIG. 9. A spool of fibers or strands (12) is woundaround a winding frame (10) of arbitrary shape. The frame (10) can havea rotation axis (11 a) in an arbitrary orientation and be rotated to analternative rotation axis (11 b) for production of bias windings. Astiffener, e.g., a thin layer of eutectically bonded fibers may beinserted between the fibers on opposite sides of the frame (10).

If desired, fibers or strands may be made into nested concentric spiralsas is shown in FIGS. 12a-12 e. To create nested concentric spiraledbrush stock of one single handedness, e.g. left-handed, for example, onemay begin with a layer of copper fibers or strands which is wound on aframe (10) as shown in FIG. 12a. The angle of the fibers (α) could beanywhere from 1 to 80 degrees or so, limited only by what can bemechanically produced, but is most suitable in the range between 5 and40 degrees. Next, one may place a silver leaf (e.g. 0.5 μm thick) on thefibers or strands and roll the fibers or strands, (FIG. 12c), into acigarette-shaped brush stock (FIG. 12d ), albeit, in commercialproduction the cigarette shaped brush stock could be indefinitely long.As shown in FIG. 12d, all of the fibers or strands will spiral aroundthe “cigarette axis in the same sense”, thus creating nested spiralconcentric spiraled fibers all of same handedness, i.e. left-handed inFIG. 12d. This configuration of fibers combines a minimum number ofcontact points (joints), i.e., low internal friction and therefore goodindependent flexibility of fiber ends, with excellent elasticcompressibility in a direction of a brush stock axis. In order to reduceor avoid the already discussed tendency of the brush stock to twist onbrush force application, two or more layers with opposite fiberinclinations may be rolled up together, characterized by the bias anglesα and β as shown in FIG. 12e, to obtain concentric layers of spiralswith alternating handedness. Note also that such nested spirals(cigarette-shaped) can be combined in parallel arrangements to formlarger diameter brush stock of arbitrary cross section as is shown inFIG. 5b. After rolling the fibers or strands into the cigarette-shapedconfiguration, a surface treatment may be needed to keep the brush stockfrom unrolling and to keep individual brushes which are cut from such abrush stock from unrolling. However, by heating to the eutectictemperature of copper and silver, for example, or mildly below, thesilver will dissolve in the copper fibers thereby hardening them, andthen the fibers will recrystallize during annealing, thereby fixing theshape of concentric spirals. Or else, with any fiber materialwhatsoever, the shape may be fixed simply through holding at therecrystallization temperature until recrystallization is substantiallyor entirely complete. As a result, depending on the particular treatmentchosen, a brush stock which is elastic, composed of hard fibers, anddoes not need a surface treatment can be achieved. Other materials maybe used besides copper and silver leaf, as was used in the example ofFIGS. 12a-e.

d) Selective Grading of Bonded Joints

Decreased distances between joints in the brush stock periphery willstrengthen it relative to that in the interior, and as a result willincrease stiffness against bending. Bonded joints can be givenpredetermined values by the use of twisted strands from tight twistingof multiple strands of the kind in FIG. 4n together, up to using onlyuncrimped fibers in the center with only as much twisting, roping orspiraling as may be needed to prevent the interior fibers from bunchingtogether. Joint spacings along the length of any one fiber or twistedstrands can thereby be graded from one or a few fiber diameters to oneinch or more.

e) Use of Support Fibers

Mixing of “support fibers”, meaning fibers of substantially greaterstiffness than the majority of the fibers into the brush stock,uniformly or with any desired gradation or distribution, willcorrespondingly mechanically strengthen the brush stock. For example,FIG. 6 shows support fibers (9) and ordinary fibers (8) in an unloadedstate. Support fibers may be of the same material as the regular brushfibers but thicker, or they may be of any suitable material includingnon-metals such as graphite, or may even be nonconducting; they may bestraight, crimped, spiraled, waved, etc., all as may be deemed to bemost suitable for imparting macroscopic strength to the brush stock withoptionally the smallest possible interference with individual fibermobility or largest macroscopic brush stock elasticity in the directionof the brush stock axis. When a brush force is applied, the supportfibers should touch the rotor or substrate surface only lightly.

Other strengthening through geometrical arrangement of the fibers cantake the form of grading the packing fraction from a high level (perhapsas much as 70%) about the periphery to a much lower value in theinterior, such as, for example, a packing fraction 15% greater on thesurface than in the interior. Alternatively, one may produce asystematic variation of two different fiber types (i.e. a slow increasein amount of one relative to the other of different material, wavinessand/or thickness) from the periphery to the center of the brush stock,e.g., so as to increase the density of bonding points progressing fromthe brush stock axis outward.

Surface Treatments

Surface treatments are used for any of the following purposes: Toprevent the unraveling of fiber arrangement at the working surface andabout the brush stock surfaces; to fix the geometrical shape of thebrush stock; to mechanically strengthen the brush stock against bending;to insulate the brush stock and the brushes cut therefrom,—from thesurroundings, including from electrical contact, physical or chemicalcontamination, or magnetic fields.

In addition to the already mentioned surface strengthening methodsthrough gradation of fiber geometry and/or strengthening of joints, thefollowing are methods to stiffen the brush stock by means of surfacetreatments which may be applied to part or all of the brush stocksurfaces:

a) the use of a sheath or casing surrounding the bulk of the fibers, asis shown in FIG. 3b, FIG. 3c, FIG. 3d and FIG. 8b.

b) wrapping the outer surface

c) Spraying, dipping, electroplating, electrophoresis, plasma sprayingand irradiation

d) stitching, as is shown in FIG. 10.

a) Casings

Strengthening through surface treatment may be achieved, through fillingan independent casing with bundled, twisted, spiraled, kinked, braided,woven, roped or felted, or a combination of any of these, fibers orstrands according to the pertinent points above. A casing of anypredetermined shape and size may be made of fibers which areeutectically bonded or be made through alloy shape fixing orrecrystallization fixing. For example, FIG. 3d depicts a triangularshape casing and FIG. 8b a rectangular shape casing.

b) Wrapping

Successful forms of mechanical strengthening via surface treatmentsinclude wrapping the fibers, with foils, strips, felt or fibers in anycombination and fastening the wrapping in any number of ways. Fasteningcan be done, for example, by an additional wrapping of a thin foil oftin or indium and briefly heating, including up to the melting point ofthe lowest-melting component.

The dimensions and kind of wrapping material may be freely chosen,constrained only by the requirements that the rotor surface not sufferunacceptable damage through the wrapping or be covered by a residuewhich interferes with the brush operation in an unacceptable manner,e.g. through increasing the film resistivity or the coefficient offriction. Conversely, the wrapping may be used to aid in a brushoperation, e.g. through containing some lubricant or mild abrasive. Inthe cases of strips and fibers, the individual turns may be inclinedrelative to the brush stock longitudinal axis at any chosen angle, from90° to as shallow an angle as may still permit the wrapping to stay inplace, which depends on the degree of fiber crimping or spiraling at thesurface but will rarely be less than 20°. Favorably, such wrapping maybe done in two or more thin layers of fibers or matted fibers,alternatively biased in orientation, e.g., ±45° inclined against thebrush stock longitudinal axis, or it may be done with thin metal foil ormetal leaf. In either case, alloy shape fixing, soldering or eutecticbonding may be used to obtain additional strengthening, or in the caseof wrapping with a metal leaf followed by annealing the only significantstrengthening that is obtained.

The inventors have successfully used indium or tin foil in combinationwith copper, silver and brass fibers, besides silver leaf and thealready indicated choices of copper or silver foil. They do not doubtthat besides brass other copper alloys including bronzes and monels willbe suitable.

c) Spraying, Dipping, Electroplating, Electrophoresis and Irradiation

Other surface treatments, some of which have been used with varyingdegrees of success, include spraying the brush stock, e.g. with a slurryof metal powder or flakes or graphite or any suitable semi-conductor, ormild abrasive or other surface conditioner. These slurries may bethickened, or caused to set in place either on natural aging orsubsequent mild heat treatment, by an admixture of agar-agar,waterglass, or cornstarch, or such liquids which have the effect ofgluing fibers in place. Any of the latter may be used with or withoutthe addition of graphite or other powders or flakes. The application ofthese surface treatments may be similarly achieved by dipping the brushstock into any of the above liquids. Should it be desired to treat onlypart of the brush stock surace, the remainder can be temporarily masked.Alternatively, more viscous constituents than may be applied throughspraying or dipping may be applied through rolling the brush stock inthem, e.g. as would apply to various powders, or slurries of the samekinds as already enumerated above. Enriching the brush stock surface bya powder or dough, e.g. by rolling or patting, could perhaps be assistedby application of a pressure difference between the inside and outsideof the intended brush to speed up the process or in order not to damagethe fiber arrangement.

Very importantly, too, surface treatment may be applied by thermalspraying including plasma spraying, flame deposition or other. Also usedmay be electroplating or electrophoresis, by which joints can be setinto place and voidage be reduced at the surface at about roomtemperature and therefore without annealing the fibers. For example,electro copper plating of copper fiber brush stock would selectivelystrengthen the surface with little other effect. One of the goals ofsurface treatments, namely protection from contaminants, and as partthereof from chemical attack, could be effected through gold plating.Electrophoresis can have especially good applicability on account of thewide range of substances which can thereby be deposited on brush stocksurfaces.

Joints can also be welded together, and new joints be created, throughlocal melting at the surface. One method for this is use of ahigh-frequency furnace, another important one is irradiation throughlasers.

e) Stitching

Stitching in the manner used for textiles or making shoes, for example,may be used for internal bonding or as one form of “surface treatment”.Stitching may be employed in lieu of, or complementing other forms of,internal bonding or surface treatment and be applied before or afterother surface treatments or eutectic bonding or alloy shape fixing, ifany. For example, FIG. 10 shows a method of stitching used to stiffenthe brush stock or individual brush (1). The threads (17) in suchstitching are typically single metal fibers or strands of metal fibersand by the proper choice of thread material relative to the fibermaterial may be set through eutectic bonding or alloy shape fixing.Stitching can be in any orientation, be distributed over the whole brushor concentrated where needed, e.g. near the running surface. The threadcan be single fibers or stands, whether twisted or not.

Ordinarily, all of the above treatments are used, or are contemplated tobe used, on brush stock or brushes not covered by a casing, butoptionally they can also be used on a casing before or after insertionof the fibers.

It may be noted that surface treatments by any of the above means, onpart or all of an outer layer and/or a component in the outer layer, maybe used temporarily, to be removed before completing the brushconstruction or just before brush use. Such removal may be donemechanically, through dissolution, etching or other means. It is furthernoted that the “surface treatment” may be used on any part(s) which areassembled into the final brush. For example, in a set of brushesconstructed by the inventors, parallel layers of fiber material wereinterspersed with thin foils.

Rotor Surface Conditioning Through Void Fillers

In one embodiment of the present invention, all or part of the voidspace is filled with a suitable material, mostly injected in the form ofa slurry of any of the kinds already enumerated in relation to dipping,spraying and rolling for surface treatments, which then solidifies inplace. The result is a considerable strengthening of the brush stockwhich may be desired in case of rather low packing fractions. Graphitefillings of this kind have been successfully used to protect the rotorsurface against oxidation (especially so far of copper fibers sliding ona silver surface and of silver fibers sliding on copper surface) whenoperating in the open atmosphere. Other useful fillers are possible.Besides graphite, candidate materials include MoS₂ and related sulfides(i.e. molybdenites) which, like graphite, provide lubrication and areelectrically conductive but should best be used in dry conditions sinceMoS₂ is attacked by moisture.

Optionally polishing agents or mild abrasives for cleaning the rotor orother surface on which the brush slides may be added to those partialvoid fillers, or they may be used alone in the same manner, albeit inonly small concentrations in order not to damage the surface and not toleave an insulating deposit. Choices of such admixtures, in anycombination, include aluminum oxide, silicon carbide, colloidal silicaand diamond powder, either alone or mixed with the already discussedfillers.

A drawback of void fillers is that they strongly reduce the fiber-endmobility on which good fiber brush operation depends, with this increaseof interior friction rising steeply with increasing fraction of voidagefilled. Interior lubrication, by contrast, can be achieved throughrinsing with a lubricant. This could be a thin oil in case theaccompanying reduction of contact resistance can be tolerated, or can bea dilute solution of colloidal graphite which is effective withoutnoticeable increase of brush resistance. Other suitable lubricants maywell exist and are being actively looked for.

Mechanical Means of Bonding or Strengthening Fiber Joints

In addition to the various means already mentioned, bonding at touchingpoints may be achieved through compacting, say in a rolling mill or“turks head” and subsequent annealing. Since compacting is incompatiblewith voidage, it requires use of a temporary matrix material which iseventually removed. The introduction of a temporary matrix material is atime consuming complication and is applicable to only a restricted rangeof matrix/fiber materials combinations.

Under clean conditions rigid fiber joints may be made through diffusionbonding without compacting.

The Role of Humidity

The presence of absorbed water layers on the contact surface is highlydesirable to prevent sticking and prolong wear. With brush materialswhich do not oxidize in the open atmosphere, normal atmospheric humidityis sufficient at low and medium current densities. Otherwise, moisturehas to be provided. The provision of adequate moisture for metal fiberbrushes, as needed, is therefore another aspect of the presentinvention.

The ambient humidity needed rises with the percentage of the rotor orsubstrate surface which is covered by brushes and also with the localheating, i.e. the current density. Normally, on continuous slip rings orrotors gaps have to be left between the brushes to permit moistureaccess. In extreme cases, moisture and/or cooling may have to be fedthrough the brushes themselves, either through the brush voidage or,given suitable fibers, through channels in some or all of the fibers.“Support fibers” will be particularly suitable for this purpose.

Miniature Brushes

For most applications, fiber brushes will be mid-sized, e.g. withcharacteristic dimensions between 0.5 cm to 3 cm. Miniature brushes madeof brush stock in the form of flat shaped strip are a further aspect ofthe present invention. Any of the already discussed considerations applyexcept for the small dimensions, easily down to ¼ mm.

Large-Sized Applications of the Fiber Brush Technology

On the other end of the scale, large-sized metal fiber brush stock canbe used for robust, long wearing, highly efficient cabling and slidingelectrical connections which can be customized for particularapplications and easily constructed with simple equipment. Specifically,flexible cables suitable for carrying currents up to hundreds of amperes(e.g. as may be needed for the rapid charging of future electrical carsor for current contacts for electric trains) could be made of brushstock, insulated from the outside, optimally composed of 50 μm orthinner metal fibers, with packing fractions in the order of f=10% orless, and a minimum of touching points and lubrication for reducedinternal friction.

Alternatively or in combination with bundled fibers, thin layers offiber felt, composed of long fibers oriented preferentially parallel tothe direction of intended current flow, can be used. Similarly, anarticulated bus (i.e. a movable jointed current conductor) for providinghigh currents to different locations could use this technology. Theencased fiber masses, of average hair-fine diameters and therefore quiteflexible, avoid the need for high forces. In addition or alternatively,the joints can be appropriately fully or partially covered with a metalfiber velvet or metal fiber felt to provide for low contact resistanceacross the relatively moving parts of any one joint, even while keepingthe friction forces low to make the joints easily rotatable. With properconstruction, the fiber felt or velvet could be made easily replaceablewhen necessary. In general, fiber felts consist of a thin layer ofmutually misoriented fiber material, bonded at a suitable concentrationof touching points, optionally without a preferential fiber direction tomake the felt equally electrically conductive in any orientation withinthe felt. A fiber velvet has much the same construction, and should bemade in much the same manner, as textile velvet, except that provisionmay be made for bonding some or many of the fiber joints for improvedelectrical conductivity.

Electrical brushes for both rotating and linear actuating applicationscould be constructed out of bundled fibers, fiber felts and/or fibervelvet, thereby providing high current capabilities, low loss and lownoise. Fiber felts or velvets can be retrofitted into existing machinerywhen desired. High power, low voltage, high-current motors areparticularly good candidates for this technology, as are signal-criticaldevices such as rotating antennae slip rings, microphones, videocameras, and other electronic and electrical devices.

Also, electrical contactors could greatly benefit from a layer of thisfelt on one of the contacting surfaces, especially when connected in thenon-energized condition. An example of this would be battery contactorswhich could charge a battery bank from a low voltage, high currentoperating configuration by connection to a high voltage configurationfor charging.

Expected Uses of Fiber Brushes

Fiber brushes are based on the theory disclosed in U.S. Pat. Nos.4,358,699 and 4,415,635 and further developed in the paper “ElectricalFiber Brushes—Theory and Observations”, by D. Kulmann-Wilsdorf,ICEC-IEEE Hohm 95 (41st. Holm Conference on Electrical Contacts, IEEE,Montreal, Canada, Oct. 2-4, 1995), pp.295-314, reprinted as “ElectricalFiber Brushes—Theory and Observations”, D. Kuhlmann-Wilsdorf, EEE Trans.CPMT Part A, 19 (1996) pp. 360-375, which is incorporated by referenceherein. This is the general theory controlling current as well as heattransfer across interfaces, at rest or in relative motion, and thedisclosed construction optimizes the conditions at the interface on amicroscopical scale. The applicability of fiber brushes is thereforeunrestricted in regard to size above the dimensions of single contactspots, as to sliding speed subject to the limitations only ofaerodynamic and hydrodynamic lift, in regard to temperature restrictedonly by the requirement that the fibers remain solid, and in regard tocurrent and heat density only to that at which the interface locallymelts. The fiber brushes are therefore applicable to all conceivablesituations of current or heat conduction across interfaces, includingrotating and reciprocating motions, as well as indefinite sliding on one(e.g. rails) or two-dimensionally extended substrates. The fiber brushestherefore, also, will in the future make possible technological orscientific developments which are still unanticipated or at the momentare stymied for lack of adequate means of current and/or heatconduction.

Specifically in terms of applications which are known at present, fiberbrushes have for example utility in electrical power equipment, inelectronic equipment especially in light of the superior signalcharacteristics as well as the capabilities presented for multiple closeproximity sliding contacts, in electric automotive applications, inpower generation and distribution systems, and in electrical linearactuators.

Methods to Control Fiber Kinking

An important aspect of the present continuous metal fiber brushconstruction is the use of kinked fibers. FIGS. 3a and 3 b are examplesof fiber brush made using kinked fibers. The desired elastic resistanceof the fiber bundles against close-packing is thereby created viamultitudes of mutual friction points of local joints (whether or notsoldered together through eutectic bonding) among neighboring fibers.The density of kinks per unit length of fiber is used to control the“loft” of the bundles. For 50 μm diameter fibers, kinks have been usedfrom a continuous spacing, i.e. making the fibers to be “waved” withdifferent amplitudes and wave lengths, to sharp kinks spanning a fewmillimeters length each spaced nearly 2.5 cm apart, and the amplitudecan be varied from fractions of a millimeter to a few millimeters. Forpractical reasons in one embodiment of this technique, the inventorshave used V-kinks and have controlled the depth of the kinks viaspooling the fibers under pre-selected tension. Hereby low tensionprovides deeper kinks while higher tension provides more shallow ones.However, it is also the case that a wide range of other kink shapes aswell as continuous kinking, e.g. in a saw-tooth pattern, an undulatingpattern, a waving or “lazy” spiraling of the fibers can be similarlyused, and that depth of initial kink profile can be used instead ofspooling tension. For mass-production, kinking, curling, spiraling etc.,applied to strands, before or after twisting, if any, whether incontinuous tows or finite lengths, instead of kinking spooled individualfibers, is also possible, and indeed will in a majority of cases be morecost effective.

Fiber Brush Stock Shaping

Fiber brushes of the present invention, other than obtained byspiraling, twisting or roping, have been made in the laboratory bycompressing the fibers in a form to yield the intended brush stock shapeand packing fraction, with or without annealing, whereby the chosensurface treatment can be either applied, or if already applied be “set”,at the same time. The forms used in the laboratory include, for example,at least once piece providing a cavity of the intended shape of thebrush stock and a matching lid by which compression can be applied toimpart the desired packing fraction. The brush stock forms were made ofstainless steel or graphite, but any other suitable material orcombination of materials can be used including a variety of metals andceramics, governed by the requirements (i) that they do not dissolve, orare dissolved in, the materials of the brush stock and (ii) that theform. maintain its shape independent of the annealing treatments used.Annealing treatments can be performed in the open atmosphere if thebrush stock form material is resistant to oxidation and is firmly closedin use to inhibit oxidation of the fibers. They will require aprotective atmosphere, e.g. of hydrogen, if the brush stock form and/orfiber stock materials are liable to oxidize at the heat treatmenttemperatures or if for some reason the form is not firmly closed, e.g.through leaks about the gaps between form components or the form isdeliberately left open at one or both of its ends. In addition to thepossible use of forms as indicated, extrusion, continuous rolling,continuous winding on mandrels, or reshaping is envisioned for largescale production of fiber brushes.

Cutting of Brushes From Brush Stock and Shaping Working Surfaces

A further important step in brush construction according to the presentinvention is cutting individual brushes from the “brush stock” andshaping their intended running surfaces. In some cases, especially forsmall dimensions and curved profiles, laser cutting may prove to be costeffective. Planar cuts through brush stock of a diameter which iscomparable to or smaller than the average spacing between touching spotsor joints can be made with a razor blade. For brush stock with arelatively large diameter, cutting poses a problem much like trying tocut a sponge without reducing the size of the pores in it. The problemis overcome by infiltrating the brush stock with a hardenable liquid (ifneed be at an elevated temperature), hardening it (e.g. cooling it tofreezing or curing it in case of a resin, as the case may be), cuttingthe brush stock and/or shaping the running surface with the hardenedliquid in it, re-melting or dissolving and removing the liquid (if needbe by means of a centrifuge), and finally cleaning residues from thebrush if necessary. Good results have been achieved using water, andcooling the water down to well below 0° C., either simply in the freezercompartment of a refrigerator or any lower temperature, e.g. of dry iceor liquid nitrogen, so as to reduce superficial melting at the cutsurface during cutting or shaping. Other fluids that might be usedinclude any aqueous liquids with surfactants aimed to increase wettingof the surface, low-viscosity oils, hard setting dissoluble gels, frozencarbon dioxide, i.e. dry ice, or commercial metallographic embedmentresins.

The actual cutting of the brush stock filled with some temporarily hardsubstance can be done by any conventional means but optimally should bedone with a sharp tool and speedily so as to avoid undue heating. Aftercutting and clearing the temporarily hard substance from the voids, thefibers at the cut face will typically be caked together. If so, theymust be freed through gentle abrasion, preferentially with some kind ofabrasive paper mounted on a substrate of the same shape as the intendedrotor or substrate surface.

Alloy shape fixing and solder-bonding of fiber joints via eutectics hasbeen employed in surface treatments while the fibers were encased in afiber brush form for imparting the desired brush stock shape and packingfraction. For example, intended bush stock of silver fibers orsilver-clad copper fibers was wrapped with a few turns of a 0.5 mm thickcopper foil; copper fibers were wrapped with one or a few turns ofsilver leaf of about 0.5 μm thickness or the form was lined with themetal leaf prior to inserting the fibers. The thickness of the wrappingis chosen depending on the size of the brush stock and the depth ofhardened layer desired. The forms were then heated to the requiredannealing temperature, typically in a protective atmosphere, meaning acover gas which does not contain oxygen or any chemically aggressivegas.

It is further noted that metal fiber brushes can, and commonly should,conduct much higher current densities than conventional brushes, andthey require much lighter mechanical pressure than conventional brushes.In fact, these are important advantages of metal fiber brushes, onaccount of which it is expected that in due course they will displaceconventional “monolithic”, graphite-based electrical brushes. However,for proper operation the brush force has to remain constant withinreasonably close, predetermined limits, independent of length of brushwear. This creates a problem because, 1) the constant-force springswidely used for conventional brushes are generally too stiff andinaccurate for applying constant light loads, and 2) conventionalcurrent leads capable of conducting the required high currents to andfrom the brushes, are stiff and interfere with the intended lightmechanical loading.

Furthermore, for practical mass applications, fiber brushes willeventually have to be sold/distributed in a packaged form which protectsthem from damage during storage, shipping and handling, and which isdesigned for fool-proof installation by unskilled workers, much likelight bulbs or printer cartridges.

In a preferred embodiment, the present invention further includes anovel electrical brush holder and loading device useful for all types ofbrushes and particularly designed to maintain constant brush force whilethe brush wears. In “inexpensive” applications one makes do with spiralspring loading wherein the brush force slowly drops with wear. For moredemanding applications one uses “constant force springs”. These aregenerally reliable but far from ideal. In preferred embodiments, themechanical loading of the brushes is done hydrostatically by means of aliquid metal which at the same time is used to conduct the current toand from the brushes. In the particular design of FIG. 7a each brush(10) is firmly, metallically fastened (e.g. via a screw connection) to ametal piston (8) in a cylinder (1) which is at least as long as thebrush. On the side of the piston away from the brush, the cylinder isfilled with the pressurized liquid metal (6). Such a combination of apiston whose end is designed for the attachment, e.g., by anelectrically conducting brush attachment(11) which can be released, of abrush and the cylinder in which it glides constitutes a “brush holder”.It may be advantageous to use a piston liner (9) and/or a cylinder liner(7) for insulation or low friction. Alternatively, the piston andcylinder may be replaced by bellows, not necessarily made of metalexcept for the provision of a conductive plate between liquid metal andthe brush.

If the over-pressure in the liquid metal is D_(P), the force exerted onthe brush will be P_(b)=A D_(p), minus the typically negligible frictionbetween piston and cylinder. Here A is the cross-sectional area of thecylinder or bellows of whatever shape, albeit presumably in most casesof circular cross-section. When the liquid metal over-pressure is keptat a constant value, the same brush force will be maintained while thepiston advances in the cylinder as the brush wears, independent of wearlength, or will drop only slowly in case bellows are used.

The open end of the cylinder may be shaped to conform, with apredetermined clearance (12), to the running surface on which the brushslides, e.g. slip ring, commutator or rail (15). Similarly, a guide maybe used in conjunction with bellows. Depending on conditions, e.g. inconnection with fast-moving vehicles, it may be advantageous to makethat clearance small so as to shield the brush from wind forces.Similarly, in motors or generators, it may be possible to shield thebrushes from magnetic forces via a ferromagnetic cylinder or coverage(16).

Preferably, the holder cylinder or bellows are provided with a stop tolimit the advance of the piston or bellows and thereby set a minimumbrush length so that the contact surface (e.g., a rotor) is protectedfrom scratching or gouging by the piston or the end of the bellows inthe event that the brush inadvertently wears out before being replaced.

In a machine or other device which requires more than one, and perhapshundreds of brushes, any selected group of brush holders may beconnected to the same liquid metal reservoir. In fact, since the brushforce is proportional to the cylinder or bellows cross sectional area,and this should ordinarily be close to, though larger than, that of thebrushes, sets of brushes of the same general construction, and thus sameelastic/plastic transition pressure, but with arbitrary shapes and sizescould be connected to the same reservoir.

Suitable bellows or hydrostatic cylinders and pistons are eitherdirectly available commercially or can almost certainly be procured frommanufacturers since bellows and hydrostatic pressure cylinders in agreat variety of shapes and sizes are manufactured in large numbers andby several firms both domestically and elsewhere. For storage, sale andhandling, the fiber brushes may be packaged in light metal or plastictubes. These should be suitably matched to the corresponding cylinder orbellows ends. Various mechanical mechanisms can be employed to fastenthe brushes to the pistons, e.g. by sliding into a dovetail while thepiston end slightly protrudes from the piston, or by a screw and threadarrangement. And similar connections can be made to the ends of bellows.Depending on construction, one or two simple valves (5) to controlaccess of the fluid to a cylinder or bellows during brush installationmay be helpful. For brush installation it may be similarly necessary topermit the cylinder or bellows to slide or swivel away from the runningsurface. This can be readily accomplished by the use of flexible plastictubing (4) for the liquid metal, for example. In any event, the currentis to be conducted through the liquid metal. An optional flexible hose(13) for the supply of moisture, lubricant, protective atmosphere,coolant, etc., or for exhaust purposes may be useful. The flexible hose(13) can be attached to the cylinder by an inlet (18). An optional valve(14) to control the access of lubricant, coolant, etc., may also behelpful. Further, a release or joint (3) may be used for easier brushinstallation. Likewise, a release or joint (17) for release of the hose(13) may be used for easier brush installation. In order to keep thecylinder in a fixed position relative to the slip ring, commutator,rail, etc. (15), a releasable or jointed attachment (2) can be used.

The most likely choices for the liquid metal are mercury (Hg) andsodium-potassium potassium alloy (NaK). Each have their advantages anddisadvantages. In view of environmental considerations, NaK ispreferred, especially since much experience with this liquid alloy isalready available. Metals melting modestly above room temperature mayalso be used, such as gallium, provided that there are means to heatthem before or immediately at the onset of use.

In addition, as depicted in FIG. 7b, there is a brush holder which makesuse of an elastically bent brush stock (1) fed through a guide (7)towards a substrate (4) so as to let it's own elastic compression serveas a brush load. FIG. 7c depicts still yet another embodiment of thepresent invention in that a brush holder has a flexible brush stock (1),a shell (5) used to contain the brush stock, a rotatable conductiveconnection (2), and connection to power (3). In addition, a fastener (6)is used to secure the shell containing the brush stock. The brush stockis guided through an opening (7) in the shell (5) towards the substrate(4). FIG. 7d illustrates an example of a guide (7) that can be used inthe brush holder of FIG. 7c. Alternatively, the rotatable brushconnection (2) can be omitted and instead the inlet end of the brushstock be directly connected to the power (3), preferably after one ormore complete turns of the brush stock (1) within the shell (5) andincluding a suitable elastic twist be imparted to the brush stock so asto force the working end of the brush stock through the guide (7)against the substrate surface (4).

Particularly advantageous in the present invention is that minorcontaminations in the liquid metals which would make them unsuitable ifused in direct contact with the rotor or slip ring surfaces, should beeasily tolerable. Moreover, the total amount of liquid metal used can bekept relatively small, and the liquid metal flow rates will be low toimperceptible even in large systems in which many brushes might beoperated simultaneously.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

What is claimed is:
 1. A brush stock for an electrical fiber brush,comprising: plural conductive elements including at least one of pluralconductive fibers and plural conductive strands of fibers; and saidconductive elements having contacting engagements with each other atirregularly longitudinally spaced contact points with the contactingengagements maintaining elastic stresses between said conductiveelements and maintaining irregularly longitudinally extended voidsbetween said conductive elements.
 2. A brush stock for an electricalfiber brush, comprising: plural conductive elements including at leastone of plural conductive fibers and plural conductive strands of fibers;and said conductive elements having contacting engagementsinterconnected by longitudinally extending fixed in shape segments ofsaid conductive elements so as to maintain irregularly longitudinallyextended voids between said conductive elements.
 3. The brush stockaccording to claims 1 or 2, further comprising: at least one of an outersurface layer, a casing, and a sheath covering at least a part of asurface of said brush stock.
 4. The brush stock according to claim 3,wherein a mechanical strength per unit area of said at least one of saidouter surface layer, said casing, and said sheath exceeds by at least15% an average mechanical strength per unit area of the conductiveelements and said voids adjacent to said at least one of said outersurface layer and said sheath.
 5. The brush stock according to claim 3,wherein said at least one of said outer surface layer, said casing, andsaid sheath differs from the conductive elements adjacent to said atleast one of said outer surface layer, said casing, and said sheath inchemical composition.
 6. The brush stock according to claim 3, wherein amechanical stiffness of an average conductive element in said at leastone of said surface layer, said casing, and said sheath is at least 10%larger than that of corresponding conductive elements adjacent to saidat least one of said outer surface layer, said casing, and said sheath.7. The brush stock according to claims 1 or 2, comprising: stitchingprovided between said conductive elements so as to fix a shape to saidbrush stock.
 8. The brush stock according to claim 7, wherein saidstitching comprises metal fibers.
 9. The brush stock according to claims1 or 2, further comprising: said brush stock having an average packingfraction f, defined as the ratio of the total cross-sectional area ofsaid conductive elements relative to the total cross-sectional area ofthe brush stock, within a range of 2% to 70%.
 10. The brush stockaccording to claims 1 or 2, comprising: said conductive elements havingbends which define at least one of a regular or irregular spiralpattern, a regular or irregular wavy pattern, a regular or irregularsaw-tooth pattern, a regular or irregular triangular pattern, a regularor irregular rectangular pattern, and a regular or irregular undulatingpattern along a length of said conductive elements.
 11. The brush stockaccording to claim 10, wherein said bends are spaced at intervalsgreater than five diameters of said conductive elements along the lengthof said conductive elements.
 12. The brush stock according to claims 1or 2, wherein said conductive elements have a diameter less than 0.2 mm.13. The brush stock according to claims 1 or 2, wherein said conductiveelements comprise a material selected from the group consisting of atleast one metal, at least one form of carbon, at least onesemiconductor, and at least one form of plastic.
 14. The brush stockaccording to claim 3, wherein said at least one of said outer surfacelayer, said casing, and said sheath comprises an average packingfraction which is greater than an average packing fraction of theconductive elements adjacent to said at least one of said outer surfacelayer, said casing, and said sheath.
 15. The brush stock according toclaim 3, wherein said outer surface layer comprises an infiltratedmaterial.
 16. The brush stock according to claim 15, wherein saidinfiltrated material is selected from the group consisting of a metal, alubricant, and an abrasive.
 17. The brush stock according to claim 3,wherein said at least one of said outer surface layer, said casing, andsaid sheath comprises at least one of a foil and a metal leaf.
 18. Thebrush stock according to claim 3, wherein said at least one of saidouter surface layer, said casing, and said sheath comprises at least onemember selected from the group consisting of a foil strip, a metal leafstrip, and a metal fiber wrapped around the brush stock at least once.19. The brush stock according to claim 17, wherein said foil is at leastpartly made of a metal.
 20. The brush stock according to claim 19,wherein said metal comprises at least one of cadmium, copper, indium,iron, nickel, niobium, tin, a noble metal, cadmium alloy, copper alloy,indium alloy, iron alloy, nickel alloy, niobium alloy, a noble metalalloy and tin alloy.
 21. The brush stock according to claim 18, whereinsaid foil strip is at least partly made of a metal.
 22. The brush stockaccording to claim 21, wherein said metal comprises at least one ofcadmium, copper, indium, iron, nickel, niobium, tin, a noble metal,cadmium alloy, copper alloy, indium alloy, iron alloy, nickel alloy,niobium alloy, a noble metal alloy and tin alloy.
 23. The brush stockaccording to claim 18, wherein said metal fiber comprises at least oneof cadmium, copper, indium, iron, nickel, niobium, tin, a noble metal,cadmium alloy, copper alloy, indium alloy, iron alloy, nickel alloy,niobium alloy, a noble metal alloy and tin alloy.
 24. The brush stockaccording to claim 3, wherein said at least one of said outer surfacelayer, said casing, and said sheath comprises at least two fibersalternatively wrapped around said brush stock at different orientations.25. The brush stock according to claim 24, wherein said orientationscomprise angles between ±20 degrees and ±90 degrees relative to a brushstock longitudinal axis.
 26. The brush stock according to claim 3,wherein said at least one of said outer surface layer, said casing, andsaid sheath comprises at least two foil strips alternatively wrappedaround said brush stock at different orientations.
 27. The brush stockaccording to claim 26, wherein said orientations comprise angles between±20 degrees and ±90 degrees relative to a brush stock longitudinal axis.28. The brush stock according to claim 24, wherein said at least twofibers comprise fibers selected from the group consisting of cadmium,copper, indium, iron, nickel, niobium, tin, a noble metal, cadmiumalloy, copper alloy, indium alloy, iron alloy, nickel alloy, niobiumalloy, a noble metal alloy and tin alloy.
 29. The brush stock accordingto claim 24, wherein said at least two fibers comprise fibers platedwith a metal.
 30. The brush stock according to claim 3, wherein said atleast one of said outer surface layer, said casing, and said sheathcomprises a predetermined size and shape so as to fix a shape to saidbrush stock.
 31. The brush stock according to claims 1 or 2, whereinsaid contacting engagements of said conductive elements comprise bondedcontacting engagements formed by at least one of the group consisting ofsoldering, welding, electroplating, electrophoresis, plasma spraying,thermally spraying, irradiation and heating said contacting engagements.32. The brush stock according to claim 3, wherein said at least one ofsaid outer surface layer, said casing, and said sheath comprises bondedcontacting engagements within a peripheral layer of said brush stockformed by at least one of the group consisting of soldering, welding,electroplating, electrophoresis, plasma spraying, thermally spraying,irradiation and heating said contacting engagements.
 33. The brush stockaccording to claims 1 or 2, further comprising: a filler materialbetween said conductive elements.
 34. The brush stock according to claim33, wherein said filler material comprises at least one of astrengthening material, an abrasive material, a lubricating material,and a polishing material.
 35. The brush stock according to claim 34,wherein said filler material is selected from the group consisting ofgraphite, MoS₂, metal, semiconductor, plastic and any mixtures thereof.36. The brush stock according to claim 34, wherein said lubricantcomprises at least one of an oil and a solution of a colloidal graphite.37. The brush stock according to claims 1 or 2, further comprising:support fibers substantially more rigid than said conductive elementsmixed within said conductive elements and mechanically strengtheningsaid brush stock.
 38. The brush stock according to claims 1 or 2,wherein said conductive elements comprise at least one of a cadmiumfiber, a cadmium alloy fiber, a copper fiber, a copper alloy fiber, asilver fiber, a silver alloy fiber, a silver-plated copper fiber, asilver-plated copper alloy fiber, a cadmium-plated silver fiber, agold-plated copper fiber, a gold-plated copper alloy fiber, acopper-plated silver fiber, a copper-plated silver alloy fiber, a goldfiber, a copper-plated gold fiber, a silver-plated gold fiber, anickel-plated gold fiber, a copper-plated gold alloy fiber, asilver-plated gold-alloy fiber, a nickel-plated gold alloy fiber, anickel-plated copper fiber, a nickel-plated copper alloy fiber, rhodiumplated gold fiber, a rhodium plated gold alloy fiber, a platinum platedcopper fiber, a platinum-plated copper-alloy fiber, a zirconium-platedcopper fiber, a chromium-plated copper fiber, and a gold-nickel-platedcopper fiber.
 39. A brush stock for an electrical fiber brush,comprising: plural conductive elements including at least one of pluralconductive fibers and plural conductive strands of fibers; and saidconductive elements having bonded contacting engagements with eachother, said bonded contacting engagements irregularly spacedlongitudinally and maintaining longitudinally irregularly extended voidsbetween said conductive elements.
 40. A brush stock for an electricalfiber brush, comprising: plural conductive elements including at leastone of plural conductive fibers and plural conductive strands of fibers,wherein plural of the conductive elements have longitudinally spacedfixed in shape segments; and said conductive elements having irregularlylongitudinally spaced bonded contacting engagements interconnected atsaid fixed in shape segments of said conductive elements to maintainlongitudinally irregularly extended voids between said conductiveelements.
 41. In a method of making a brush stock for an electricalfiber brush, the improvement comprising: obtaining plural conductiveelements including at least one of plural conductive fibers and pluralconductive strands of fibers; and arranging said plural conductiveelements in contacting engagement with each other at irregularlylongitudinally spaced contact points with the contacting engagementmaintaining said conductive elements under elastic stresses to maintainirregularly longitudinally extended voids between said conductiveelements.
 42. In a method of making a brush stock for an electricalfiber brush, the improvement comprising: obtaining plural conductiveelements including at least one of plural conductive fibers and pluralconductive strands of fibers, and plural of said conductive elementshaving longitudinally extending fixed in shape segments; and arrangingthe obtained plural conductive elements with the fixed in shape segmentsof different of said elements irregularly spaced with respect to oneanother in contacting engagement interconnected by said fixed in shapesegments of said conductive elements to maintain irregularlylongitudinally extended voids between said conductive elements.
 43. Themethod of claims 41 or 42, further comprising: covering at least a partof an outer surface of said brush stock with at least one of an outersurface layer, a casing, and a sheath to maintain said conductiveelements under elastic stress.
 44. The method of claims 41 or 42,further comprising: covering at least a part of an outer surface of saidbrush stock with at least one of an outer surface layer, a casing, and asheath to provide a protective covering to said conductive elements. 45.The method of claims 41 or 42, further comprising: compressing saidarranged conductive elements in a form of a predetermined size and shapeso as to fix a shape to brush stock.
 46. The method of claim 44, furthercomprising: simultaneously heating said conductive elements whilecompressing said conductive elements.
 47. The method of claim 44 or 45,further comprising: stitching said conductive elements together so as tofix a shape to the brush stock.
 48. The method of claims 41 or 42,comprising: providing conductive elements having bends formed bycrimping, kinking, waving, spiraling, pleating, folding, and curlingsaid conductive elements.
 49. The method of claims 41 or 42, whereinsaid arranging step comprises: placing a layer of said conductiveelements on a thin metal foil; and rolling up the thin metal foil withsaid layer of said conductive elements placed thereon.
 50. The method ofclaims 41 or 42, wherein said arranging step comprises: rolling up saidconductive elements.
 51. The method of claims 41 or 42, wherein saidarranging step comprises at least one of the steps of twisting, felting,roping, matting, spiraling, braiding, interweaving and interlinking saidconductive elements.
 52. The method of claims 41 or 42, furthercomprising: partially filling spaces between said conductive elementswith at least one of a strengthening material, a lubricating material, apolishing material, and an abrasive material.
 53. The method of claim43, further comprising: heating said brush stock to a melting-pointtemperature of at least one component of said at least one of said outersurface layer and said sheath.
 54. The method of claims 41 or 42,further comprising: inserting said brush stock into a casing of apredetermined size and shape so as to fix a shape to the brush stock.55. The method of claim 43, further comprising: heating said brush stockto a melting-point temperature of an alloy formed of at least twochemical constituents of said at least one of said outer surface layer,said casing, and said sheath.
 56. The method of claims 41 or 42, furthercomprising: dipping or rolling said brush stock into a powder-mixturecomprising a constituent of a metallic eutectic; heating said brushstock to a melting-point temperature of said metallic eutectic; andcooling said brush stock.
 57. The method of claims 41 or 42, furthercomprising: spraying at least a portion of an exterior of said brushstock with a strengthening material.
 58. The method of claims 41 or 42,further comprising: heating said brush stock to induce local melting oreutectic formation at interconnections of said conductive elements. 59.The method of claims 41 or 42, further comprising: irradiating saidbrush stock to induce local melting or eutectic formation atinterconnections of said conductive elements.
 60. The method of claims41 or 42, further comprising: eutectically bonding said contactingengagements of said conductive elements.
 61. The method of claims 41 or42, further comprising: cutting a brush from said brush stock.
 62. Themethod of claims 41 or 42, further comprising: shaping an end of saidbrush stock.
 63. The method of claim 62, further comprising: slidingsaid end of said brush stock against an abrading material shaped toconform to a shape of a rotor or other substrate surface.
 64. The methodof claim 61, wherein said cutting step comprises: infiltrating at leasta portion of one end of said brush stock with a hardenable or freezableliquid; hardening or freezing said liquid; cutting said brush stock; anddissolving or melting and removing said liquid from said brush stock.65. The method of claims 41 or 42, wherein said arranging stepcomprises: mixing support fibers in between said conductive elements.66. The method of claims 41 or 42, further comprising: introducing acomponent into the brush stock; and heating said brush stock to diffusesaid component into said conductive elements.
 67. The method of claim66, wherein said component comprises at least one of a foil and apowder.
 68. In a method of making a brush stock for an electrical fiberbrush, the improvement comprising: obtaining plural conductive elementsincluding at least one of plural conductive fibers and plural conductivestrands of fibers; arranging said plural conductive elements incontacting engagement with each other; and bonding the contactingengagements such that the bonded contacting engagements are irregularlyspaced longitudinally and maintain longitudinally irregularly extendedvoids between the conductive elements.
 69. In a method of making a brushstock for an electrical fiber brush, the improvement comprising:obtaining plural conductive elements including at least one of pluralconductive fibers and plural conductive strands of fibers, whereinplural of the conductive elements have longitudinally spaced fixed inshape segments; arranging said plural conductive elements in contactingengagement interconnected at said fixed in shape segments of saidconductive elements; and bonding the contacting engagements such thatthe bonded contacting engagements are irregularly spaced longitudinallyand maintain longitudinally irregularly extended voids between theconductive elements.